Treatment of stainless steel



Patented Oct. 3, 1939 UNITED STATES TREATMENT or STAINLESS STEEL Hans 0. Kauifmann and Robert L. MeEwen, Buffalo, N. Y., assignors to Buifalo Electro-Chemical Company, Inc., Buffalo, N. Y.

N Drawing.

Application March 4, 1937,

Serial No. 129,104

3 Claims. (Cl. 148-65) of the reaction product of the solution containing active chlorine and the stainless steel exerts a detrimental effect upon the substance being treated.

It is an object of the present invention to provide a treated stainless steel that will resist the corrosive action of solutions containing active chlorine and to provide a method of treatment whereby stainless steels are rendered res stant to attack by solutions containing active chlorine.

We have found that stainless steels treated with persulfates resist the action of and are not attacked by solutions containing active chlorine from about 5 to 30% nickel, together with other alloying constituents.

such as the alkali metal hypochlorites, the alkaline earth metal hypochlorites, as well .as other hypochlorite containing solutions, solutions of chlorine in alkali and the like, all of which contain chlorine generally designated as active chlorine.

The treatment of the stainless steel by means of the persulfate may be carried out in any convenient. fashion, as, for instance, by contacting the stainless steel vessel, conduit, fitting or other article with an aqueous solution of the persulfate. The materials are permitted to remain in contact for a. time suflicient to produce upon the surface of the stainless steel article the resistant layer, skin, or surface rendering the article impervious to the action of active chlorine. In general, this contact time will vary depending upon the concentration of the persuliate solution and upon the temperature at which the treatment is effected. In general, the higher the temperature and the greater the concentration of the persulfate in the solution, the shorter the time required to produce passivity, or build up the resistance on the surface of the article.

Where a solution containing about one to two grams of potassium persulfate per liter is used to treat a stainless steel article, a contact time at room temperature of several hours to several days may be required although a contact time of about 8 to 48 hours has been found sufllcient to produce the desired surface eifect and render the stainless steel article resistant to solutions containing active chlorine.

The treatment may also be eifected at elevated temperatures and a contact time of about an hour to about 6 hours or more, when using a persulfate solution containing about 1 to 4 grams of an alkali metal persulfate, is employed. The solution may be heated to a temperature below that at which the persulfate maybe adversely affected and, in general, temperatures of about 55 to 90 C. are recommended, although the action of the persulfate upon the stainless steel article proceeds at almost any temperature.

It' will be understood the contact of the persuli'ate and stainless steel article may be effected in any convenient fashion and the article may be sprayed, immersed in, or if a vessel, the same may be filled with the solution ofv persulfate or the solution circulated in around or about the article.

Where the term stainless steel" has been used, those alloy steels are designated that contain sufficient chromium, together with other alloying elements to render the steel resistant to salt spray weathering action. Such steels normally contain from about 8 to 30% chromium and may contain What is claimed is:

1. The method of employing stainless steels in conjunction with active chlorine without attacking the steel which comprises applying a persulfate to the steel for a-time and at a temperature suiiicient to render the surface of the steel inactive to active chlorine and thereafter bringing the steel in contact with the solution containing active chlorine.

2. The method of employing stainless steel in conjunction with active chlorine, without attacking the steel which comprises treating the steel for a time sumcient and with an aqueous solution containing a persulfate in concentration suilicient to render the surface of the steel inactive to active chlorine and thereafter bringin the steel in contact with the solution containing active chlorine.

3. The method of employing stainless steel in conjunction with active chlorine without attacking the steel which comprises treating the steel at an elevated temperature with an aqueous solution of a suflicient concentration of a persulfate for a time suflicient to render the surface of the steel inactive to active chlorine and thereafter bringing the steel in contact with the solution containing active chlorine.

HANS O. KAUFFMANN. ROBERT L. MCEWEN. 

